In recent years bushing assemblies have been useful as vibration isolation and damping structure at a connecting joint between a base member and a member movable about the connecting joint in such applications as machinery, airplanes, boats and vehicular transportation. Generally, the base member has two ends, one end affixed to a relative stationary frame and the second end defining a mating member forming a part of a housing of the connecting joint. The member movable about the connecting joint also has two ends, one end affixed to an element movable with respect to the frame, and a second end defining a mating member which along with the mating member of the base member forms the housing of the connecting joint. The bushing assembly is affixed within the housing of the connecting joint and controls forces and accommodates movement from the movable element generally in the radial direction of the housing assembly.
Various forms of movements occur at the connecting joint including static and dynamic vibratory motions. It is desirable that the bushing assembly be capable of isolating and damping such vibratory motions, thereby reducing the transmittal of such vibratory motions to the base member while accommodating static deflections.
In vehicular transportion such as automobiles, bushing assemblies are widely used at joints in the suspension assemblies to control forces and accommodate movement including vibratory motions. One concern in automobiles is the reduction of vibratory motions induced from the road surface and isolation of the passenger compartment from such motions. Such vibratory motions may comprise a range of amplitudes and frequencies and mtions in various directions. Relative to the road, the vibratory motion may be vertical, such as up and down motion, or it may be horizontal, such as the sway motion incurred in cornering of a car. Automotive suspension systems are designed to reduce such vibratory motions. A front end suspension system is made up of components including various arms, rods, links, etc. intermediate of the frame and the wheel assembly of the car. Generally, an elongated arm extends from the wheel assembly, and another arm extends from the frame which are connected together at a connecting joint by a bushing assembly.
One common type of bushing assembly is a rubber bushing. An example of such a bushing is disclosed in U.S. Pat. Nos. 4,121,813 and 2,948,502. In the disclosed patents the rubber bushings generally have annular elongate inner and outer members disposed in a coaxial and radially spaced apart relation and a resilient elastomer member interposed therebetween. Such bushings are used to control and transmit movement but have limited capability in damping vibrations. Damping of vibrations is attained by dissipating the energy of the vibratory motion. The damping provided by elastomers is a function of the hysteresis property of the elastomer. In general, rubber bushings can be said to provide little damping.
Another form of bushing assembly is fluid filled bushings. Such bushings are more suitable to provide damping. An example of such bushings are disclosed in U.S. Pat. Nos. 3,698,703 and 3,642,268. In the disclosed patents, the fluid filled bushings generally include a cylindrical elongate inner rigid member, an elongate outer rigid sleeve member concentrically disposed and radially spaced from the inner member and a resilient means disposed between the inner member and outer sleeve member. The resilient elastomeric means defines a pair of circumferentially spaced and diametrically opposed fluid filled chambers fluidly connected by an elongate restricted passageway.
The fluid chambers and restricted passageway are filled with a suitable incompressible fluid. The fluid chambers are fluidly connected by the restricted passageway. U.S. Pat. No. 4,693,456 disclosed a fluid filled resilient bushing assembly having a spiral shaped restricted passageway in the elongate outer rigid sleeve.
Such fluid filled bushing assemblies are installed in connecting joints such that the longitudinal axis of the bushing is generally perpendicular to the longitudinal axis of the arms connected. In such an assembly, motions are conducted along the radial axis of the bushing. In operation, in response to vibratory motions along the radial direction of the bushing assembly between the inner member and outer sleeve member, fluid is displaced from one chamber via the restricted passageway to the second chamber in a direction opposite to the vibratory motion. In particular, when a first chamber is contracted, the fluid is displaced therefrom through the restricted passageway to an expanding second chamber. In the reverse cycle of the vibratory motion, when the first chamber is expanding and the second chamber is contracting, the fluid is reversibly moved through the restricted passageway. As can be seen, an oscillatory motion of the fluid is generated within the restricted passageway between two chambers diameter about a radial direction.
The oscillatory fluid in the restricted passageway creates a fluid resistance and/or a means or inertia resistance to the pumping forces of the chambers resulting in damping or the vibratory motions along the radial direction of the bushing assembly. The chambers may be circumferentially spaced to provide damping along more than one radial direction. Intermediate of the chambers, the bushing assembly comprises a solid rubber member, extending along the axial direction of the bushing assembly wherein these sections of the bushing assembly have the characteristics of the rubber bushing with respect to vibratory motion.
The above referred to bushing assemblies are suitable for applications having zero static radial deflection of the inner member relative to the outer member. In these applications, the radial distance between the inner and outer members is generally equal until motions are applied to deflect the inner member relative to the outer member. However, in some applications the design of the connecting arms impose a static radial deflection on the bushing assembly, offsetting the inner member relative to the outer member under static conditions. Such continuous static deflection imposes a stress on the end walls of the bushing assembly, possibly resulting in a shorter life for the bushing. It is desirous to have a bushing assembly with a preload that would oppose static deflection such that the bushing would have a generally equal radial distance between the inner and outer members under static conditions.